1. Field of the Invention
The present invention relates to rechargeable lithium batteries and more particularly to rechargeable lithium batteries which use a lithium alloying metal for both or either of the active anode and cathode materials.
2. Description of Related Art
Rechargeable lithium batteries, because of their high energy densities, have been noted to lead the next generation. The use of metallic lithium for the negative active material, although effective to increase a charge-discharge capacity, is accompanied by lithium dissolution and deposition during charging and recharging, dendrite formation at a negative electrode and electrode deformation which together cause poor battery cycle performances and thus prevent provision of practicable batteries.
As solutions to such problems, an Li alloy negative electrode using an Li alloying metal and a carbon negative electrode using a carbon material have been proposed. The carbon negative electrode has been put into practical use for years but suffers from a problem of a larger energy density drop compared to the metallic lithium negative electrode, due to its low theoretical capacity, 372 mAh/g. On the other hand, the Li alloy negative electrode is subjected to increasing pulverization with charge-discharge cycling and thus suffers from a problem of poor cycle performances.
For example, Japanese Patent Laying-Open No. Hei 10-312804 proposes a method for improving cycle performances by using a homogeneous single-phase alloy derived via formation of a metal powder by a roll quenching process and subsequent heat treatment thereof.
However, the metal powder obtained in accordance with this method consists of large-size, homogenous single-phase alloy particles which are susceptible to a large stress, during the Li alloy formation, that problematically causes active material to fall off from a current collector. Other problem has been that the metal particles tend to grow into treelike dendrites to render active material inactive. As such, those methods have even failed to obtain sufficient cycle performance characteristics.
It is an object of the present invention to provide a rechargeable lithium battery which uses an Li alloying metal as the active material of at least one of positive and negative electrodes and which exhibit excellent cycle performance characteristics.
The rechargeable lithium battery of the present invention includes a positive electrode, a negative electrode and a nonaqueous electrolyte. Characteristically, an Li alloying metal is used as the active material of at least one of the positive and negative electrodes and this metal active material is covered with a thin film which is non-reactive with Li ions, permits passage of Li ions but does not have an Li ion conductivity.
In the present invention, the provision of such a thin film over the metal active material regulates dendrite formation or pulverization of the metal active material during charge-discharge cycles and thus prevents the metal active material from falling off from a current collector.
The thin film provided to overlie the metal active material in accordance with the present invention is the one that is nonreactive with Li ions, permits passage of Li ions but does not have an Li ion conductivity, as stated above. Because of its nonreactivity with Li ions, the thin film itself is not alloyed and thus avoids the occurrence of expansion or shrinkage. Also, the passage of Li ions across the thin film permits a cell reaction to occur and proceed in metal active material. Also, the thin film itself does not undergo deformation during charging and discharging because it does not have an Li ion conductivity, as contrary to solid electrolyte thin films.
Also in the present invention, the thin film preferably has a volume resistivity of not exceeding 1010 xcexa9xc2x7cm. Such a good electronic conductivity enables the thin film to also serve as a current collector.
In the present invention, the thin film can be formed such as by a CVD, sputtering or vacuum deposition process.
The thin film in the present invention is illustrated by a hard carbon thin film such as a diamond-like carbon thin film. Such a thin film is the one that does not react with Li ions, permits passage of Li ions and does not have an Li ion conductivity. The preferred hard carbon thin film shows two peaks Id and Ig in the Raman scattering spectrum, around 1400 cmxe2x88x921 and 1550 cmxe2x88x921, with a ratio (Id/Ig) in intensity of 0.5 to 3.0.
As also described above, the preferred hard carbon thin film has a volume resistivity of not exceeding 1010 xcexa9xc2x7cm and can be illustrated by a CO2-containing hard carbon thin film with a good electrical conductivity. The CO2-containing hard carbon thin film can be formed by a CVD process using a gas mixture of CO2 and hydrocarbon as a source gas. Also, a good electrical conductivity can be imparted to a surface of a hard carbon thin film by treating the surface with a CO2-containing gas.
The thickness of the thin film is not particularly specified, but is preferably in the approximate range of 50 to 1,000 nm, more preferably in the approximate range of 100 to 500 nm. If the thin film is too small in dimension, its presence may in some cases become ineffective to prevent the metal active material from falling off from a current collector and to regulate the treelike growth of the metal active material. On the other hand, if the thin film is too large in dimension, the passage of Li ions during charging and discharging may sometimes be regulated to result in the insufficient reaction between the metal active material and Li ions.
As noted earlier, an Li alloying metal serves as the metal active material in the present invention. The Li alloying metal may be at least one metal selected from Si, Ge, Sn, Al, In and Mg. The metal active material may be rendered into a powder form and combined with a binder to form a mix which is then coated on a current collector to fabricate an electrode, in accordance with a general procedure utilized to fabricate an electrode for use in rechargeable lithium batteries. Preferably, the metal active material is used in the form of a thin film. The metal active material may be deposited in the form of a thin film by various techniques, including CVD, sputtering, vapor deposition and plating techniques. In such a case, the metal active material, if deposited on a substrate comprised of a current collector such as a copper foil, can be made into a ready-to-use electrode. Alternatively, a metal foil may be used as the film-form metal active material. In this case, the aforementioned thin film may be placed on both sides of the film-form metal active material. In the case where a metal foil is used as the film-form metal active material, the metal foil may be allowed to serve as a current collector.
In the present invention, an interlayer may be provided between the aforesaid thin film and metal active material for purposes including improvement of adhesion therebetween. Such an interlayer may be formed from at least one selected from Si, Ti, Zr, Ge, Ru, Mo, W and their oxides, nitrides and carbides. Preferably, the interlayer has a thickness in the approximate range of 10 to 500 nm. The interlayer can be deposited by various techniques, including CVD, sputtering, vapor deposition and plating techniques.
The metal active material in the present invention may serve as either a positive or negative active material, but probably in most cases as a negative active material due to its electric potential relative to Li.
If the latter is the case, any substance conventionally known to serve as positive active material of rechargeable lithium batteries can be used to constitute the positive active material. Examples of useful positive active materials include metal oxides containing at least one metal selected from manganese, cobalt, nickel, vanadium and niobium. Examples of specific metal oxides include lithium-containing transition metal oxides such as LiCoO2, LiNiO2, LiMn2O4, LiMnO2, LiCo0.5O2 and LiNi0.7Co0.2Mn0.1O2; lithium-free metal oxides such as MnO2; and the like. Other substances can also be used, without limitation, if they are capable of electrochemical insersion release of lithium.
The electrolyte solvent for use in the rechargeable battery of the present invention is not particularly limited in type but can be exemplified by a mixed solvent which contains cyclic carbonate such as ethylene carbonate, propylene carbonate or butylene carbonate and also contains chain carbonate such as dimethyl carbonate, methyl ethyl carbonate or diethyl carbonate. Also applicable is a mixed solvent of the above-listed cyclic carbonate and an ether solvent such as 1,2-dimethoxyethane or 1,2-diethoxyethane. Examples of electrolyte solutes include LiPF6, LiBF4, LiCF3SO3, LiN(CF3SO2) 2, LiN(C2F5SO2) 2, LiN(CF3SO2)(C4F9SO2), LiC(CF3SO2) 3, LiC(C2F5SO2) 3 and mixtures thereof. Other applicable electrolytes include, for example, a gelled polymer electrolyte comprised of an electrolyte solution impregnated into a polymer electrolyte such as polyethylene oxide or polyacrylonitrile and inorganic solid electrolytes such as LiI and Li3N. The electrolyte for the recharageable lithium battery of the present invention can be used without limitation, so long as an Li compound as its solute that imparts an ionic conductivity, as well as its solvent that dissolves and retains the Li compound, remain undecomposed at voltages applied during charge, discharge and storage of the battery.